Effect of the loading frequency on fatigue properties of JIS S15C low carbon steel and some discussions based on micro-plasticity behavior

Abstract Ultrasonic testing method has been often used to investigate fatigue properties of various metallic materials. Since ultrasonic fatigue tests are conducted at a very high loading frequency, they are particularly convenient for fatigue tests in the very high cycle regime. Indeed, ultrasonic fatigue method allows us to conduct fatigue tests up to 109–1010 cycles in a definite period. However, due to the huge gap of loading frequency between ultrasonic testing method (around 20 kHz) and usual testing method (most of the cases in the range 1–100 Hz), the frequency effect on the fatigue property is still unclear. Low carbon steel is one of typical metallic materials to provide a significant discrepancy between fatigue strengths obtained under ultrasonic testing frequency and under usual testing frequency range. Thus, by preparing a lot of specimens of JIS S15C low carbon steel (0.15% C), fatigue tests were carried out in a wide range of the loading frequency. The frequency effect on the S–N property was first examined and a useful procedure was proposed to obtain a common S–N property normalized by the lower yield stress. In addition, micro-plasticity behavior such as the stress–strain hysteresis loop and the local misorientation were also measured and the frequency effect on the fatigue property was discussed.